1. Field of the Invention
This invention relates to an optical displacement sensor for high-sensitivity measurement of the surface shape and roughness of workpieces and the like.
2. Description of the Prior Art
The surface shape and roughness of workpieces are important factors in the evaluation of machining characteristics. Recently machining is being applied to spherical and aspherical mirrors. For this, the workpiece surface is measured and this data is fed back to the machining section. In such cases the roughness of the workpiece mirror surface may be in the order of 10 nm, for example. Therefore it is necessary to use a high precision, high sensitivity displacement sensor to measure the roughness of such a surface. In the prior art, optical displacement sensors have been developed that use critical angle focus error detection or astigmatic focus error detection to perform non-contact measurement of such workpiece surfaces, and these sensors have been applied to very high precision in-process measurement. A typical arrangement of a conventional optical displacement sensor of this type is shown in FIG. 9. Light from a laser diode 12 is reflected by a semi-reflecting mirror 4 and a mirror 6b and projected onto a target surface 14 of a workpiece by an optical probe that has a set of objective lenses 2, and the light reflected by the surface is detected by a four-piece photodiode 40. The ups and downs of surface irregularities produce a corresponding change in the position of the focal point F1 along the axis of the light beam, causing the shape of the light spot on the four-piece photodiode 40 to change shape, so the four-piece photodiode 40 measures these changes in the shape of the light to thereby detect irregularities on the target surface 14. However, the sensor cannot distinguish the direction of these changes in the position of the focal point F1, that is, whether a surface irregularity is positive or negative. To allow the direction to be discriminated, there are arrangements that use a cylindrical lens 37 whereby positional changes in the focal point F1 produce a corresponding change in the direction of distortion of the light spot on the four-piece photodiode 40.
With a conventional optical displacement sensor thus arranged, if the light beam point of reflection on the surface is at an angle .theta. with respect to the light axis, the angle of the reflected light will be 2.theta. relative to the incident light beam, so a large angle .theta. can result in the reflected light not entering the optical detection system. Moreover, there is also a tendency for information relating to target surface inclination to interfere with information relating to target surface irregularities, increasing the error in measurements of target surface irregularities. There is therefore an increasing need for an optical displacement sensor whereby, when a target surface is inclined, can apply correction so that the light reflecting from the surface passes correctly along the axis of the optical system, thereby enabling measurement to be carried out with high precision, unaffected by the inclination of the target surface.